1. Field of the Invention
This invention relates to heat-sensitive recording elements particularly useful for making color hard copy, to a method of imaging using said elements and to novel organic compounds useful as the image-forming materials in said heat-sensitive recording elements.
2. Description of the Prior Art
A variety of thermal imaging systems for producing color images have been proposed, and several have been mentioned in Kosar, J., Light-Sensitive Systems: Chemistry and Application of Nonsilver Halide Photographic Processes, New York, John Wiley and Sons, Inc., 1965, pp. 402-19. In one type of heat sensitive recording system, a first sheet containing a first reagent is superposed with a second sheet containing a second reagent and one of the reagents is melted or vaporized by the imagewise application of heat and transferred for reaction with the other reagent to form a color image. In another type of "transferring system", images are formed by sequentially transferring two or more dyes carried on separate donor sheets to a common receptor sheet by melting or volatilization. In thermal imaging systems of the "self-containing" type, a single sheet is used and the imagewise heating of the heat-sensitive sheet produces a color image, for example, by rendering a coating layer transparent to reveal the color of a background layer, by initiating the chemical reaction of two or more reagents to form a colored product or by bleaching, coloring or changing the color of a single reagent. In most of the non-silver thermal imaging systems in commercial use, color images are formed using two or more reagents that usually are encapsulated or otherwise isolated from each other until melted and mixed upon imagewise heating.
A number of compounds which undergo a color change from a colorless to a colored form, from one color to another color or from a colored to a colorless form upon application of heat have been disclosed. For example, U.S. Pat. No. 3,723,121 discloses several thermochromic materials for laser beam recording including inorganic compounds, such as, black copper (II) oxide which decomposes to red copper (I) oxide upon heating and organic compounds, such as, polyacetylene compounds which subsequent to treatment with ultraviolet light undergo two changes in color, first to red then to yellow, as the temperature is increased. U.S. Pat. No. 4,242,440 discloses another class of heat-sensitive polyacetylene compounds which exhibit color changes, for example, gold to red, brown to orange and gold to orange, which color changes are reversible. U.S. Pat. No. 3,488,705 discloses thermally unstable organic acid salts of triarylmethane dyes useful in electrophotographic elements as sensitizing dyes that are decomposed and bleached upon heating. U.S. Pat. No. 3,745,009 reissued as Re. 29,168 and U.S. Pat. No. 3,832,212 disclose heat-sensitive compounds for thermography containing a heterocyclic nitrogen atom substituted with an --OR group, for example, a carbonate group that decolorize by undergoing homolytic or heterolytic cleavage of the nitrogen-oxygen bond upon heating to produce an RO+ ion or RO' radical and a dye base or dye radical which may in part fragment further. U.S. Pat. No. 4,380,629 discloses styryl-like compounds which undergo coloration or bleaching, reversibly or irreversibly via ring-opening and ring-closing in response to activating energies such as light, heat, electric potential and so on.